Ultra-short transistor fabrication scheme for enhanced reliability

ABSTRACT

A detached drain transistor including a semiconductor substrate, a gate dielectric formed on an upper surface of the substrate, a conductive gate formed on the gate dielectric, a first pair of spacer structures, a first source impurity distribution, a second pair of spacer structures, and a drain impurity distribution. The conductive gate is laterally disposed over a channel region of the semiconductor substrate. The channel region extends laterally between a first source region of the semiconductor substrate and a detached drain region of the semiconductor substrate. A channel boundary of the detached region is laterally displaced from a first sidewall of the conductive gate by a drain displacement. A channel boundary of the first source region is laterally displaced from a second sidewall of the conductive gate by a source displacement. The first pair of spacer structures is formed in contact with the first and second sidewalls of the conductive gate. A lateral dimension of the first pair of spacer structures is approximately equal to the source displacement. The second pair of spacer structures is formed on exterior sidewalls of the first pair of spacer structures such that exterior sidewalls of the second pair of spacer structures are displaced from respective sidewalls of the conductive gate by approximately said drain displacement. In a presently preferred embodiment, the source displacement is approximately 50 to 400 angstroms while the drain displacement is approximately 500 to 1500 angstroms.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of semiconductor processing and MOStransistors and, more particularly, to a structure and method in whichsource/drain regions within the semiconductor substrate are detached orlaterally displaced from the transistor gate.

2. Description of the Relevant Art

The operating characteristics of transistors fabricated withmetal-oxide-semiconductor (MOS) integrated circuit techniques are afunction of the transistor's dimensions. In particular, thesource-to-drain current (I_(ds)) is proportional to the ratio of is thetransistor's width (W) to the transistor's length (L). For a giventransistor width and a given biasing condition (e.g., V_(G) =3V, V_(D)=3V, and V_(S) =0V), I_(ds) is maximized by minimizing the transistorlength L. Minimizing transistor channel length also improves the speedof integrated circuits comprised of a large number of individualtransistors because the larger drain current associated with a shortchannel length can drive the adjoining transistors into saturation morequickly. Minimizing L is, therefore, desirable from an device operationstandpoint. In addition, minimizing the transistor length L is desirablefrom a manufacturing perspective because a smaller area of silicon isrequired to manufacture a transistor having a smaller length. Byminimizing the area required for a given transistor, the number oftransistors available for a given area of silicon increases and with it,a corresponding increase in the circuit complexity that can be achievedon the given area of silicon.

The benefits achieved by minimizing the channel length L are accompaniedby an increased sensitivity to voltage breakdown. As device channellengths drop below 0.5 microns, the stability and reliability of thetransistor must be carefully monitored. One widely recognized and easilytested parameter is the drain voltage breakdown (commonly referred to asBVDSS). BVDSS can be measured by grounding the gate, source, andsubstrate of a test transistor and ramping the drain voltage from 0volts while measuring the drain voltage at which a drain current ofapproximately 1 μA begins to conduct. BVDSS, therefore, indicates adrain voltage at which the device conducts current whether or not a biasis applied to the transistor gate. Because unwanted drain currentsincrease the power requirements of the integrated circuit, thetemperature of the integrated circuit, and can result in an inadvertentactivation of other transistors within the circuit, the maximization ofBVDSS is critical, especially in short channel devices. In conventionalMOS transistors, BVDSS begins to approach the operating voltage of thetransistor as the channel length falls below approximately 0.5 microns.FIG. 1 shows a transistor 10 which is comprised of a substrate 12, agate dielectric 14, a gate electrode 16, and a pair of source/drainregions 20a and 20b. Transistor 10 is fabricated such that channelboundaries 22a and 22b of source/drain regions 20a and 20b are in closeproximity to lateral positions of first and second sidewalls 18a and 18bof gate electrode 16. Source/drain regions 20a and 20b are typicallyheavily doped with an impurity to provide a plentiful source of mobilecarriers for conduction after an inversion region is created in thesubstrate 12 under gate electrode 16. The use of heavily dopedsource/drain regions that have a channel boundary coincident withsidewalls of the transistor gate results in a transistor having anundesirably low field breakdown voltage BVDSS. BVDSS decreases withchannel length L partly because the maximum electric field withinchannel region 21 of transistor 10 increases. This increased electricfield can provide sufficient energy to mobile carriers within drainregion 20b to overcome the reversed biased junction between drain region20b and channel region 21 thereby increasing the drain current.

One well known approach to reduce the short channel effects described inthe preceding paragraph includes the fabrication of lightly doped drain(LDD) structures. FIG. 2 shows a typical transistor 30 incorporating LDDregions 40a and 40b. Transistor 30 includes a semiconductor substrate32, a gate dielectric 34, a gate electrode 36, lightly doped regions 40aand 40b, and heavily doped regions 48a and 48b. Transistor 30 alsoincludes spacer structures 44a and 44b that facilitate the lateraldisplacement of the heavily doped regions 48a and 48b from sidewalls 38aand 38b of gate electrode 36. Typically, the peak impurity concentrationwithin lightly doped drain regions 40a and 40b is less than the peakimpurity concentration within heavily doped regions 48a and 48b. Channelboundaries 42a and 42b of lightly doped regions 40a and 40b areapproximately aligned with lateral positions of first sidewall 38a andsecond sidewall 38b of gate electrode 36. Interior boundaries 49a and49b of heavily doped regions 48a and 48b are laterally displaced fromsidewalls 38a and 38b of gate electrode 36. Because lightly dopedregions 40a and 40b are typically doped with a lighter impurityconcentration than heavily doped regions 48a and 48b, the resistivity oflightly doped regions 40a and 40b is higher than a resistivity ofheavily doped regions. Accordingly, an applied drain voltage isdistributed across lightly doped drain region 40b and results in reducedelectric field within channel region 41 that results in an increasedBVDSS of transistor 30. It will be appreciated that in the case of bothtransistor 10 of FIG. 1 and transistor 30 of FIG. 2, the channelboundaries of the source/drain impurity distributions are approximatelycoincident with sidewalls of the gate electrode. This alignment of thesource/drain boundaries and the gate electrode sidewalls has generallybeen considered desirable. Significant overlap between the gateelectrode and the source/drain regions is avoided because of theincreased parasitic capacitance that results when a source/drain regionextends significantly below the gate electrode. Because the transistordrain typically functions as the device output and the gate electrodetypically functions as the device input, any parasitic capacitancebetween drain and gate produces an undesirable feedback mechanism thatlimits the high frequency operation of the device. See, e.g., Ben G.Streetman, Solid State Electronic Devices 319-321 (Prentice-Hall 1980).Therefore, conventional transistors have generally been fabricated in amanner designed to minimize overlap between the source/drain regions andthe gate electrode, most notably through the use of the self alignedsilicon gate technology. Despite the desire to minimize parasiticcapacitance due to excessive overlap, conventional transistor designtypically required some lateral overlap between the source/drain regionsand the gate electrode. The overlap was generally considered necessaryto form a complete channel from source to drain region and it wasbelieved that a non-functional device could result if the gate did notextend to the source and drain impurity distributions. Id.

The requirement that the gate electrode extend to the source and drainimpurity distributions results in an undesirably low BVDSS when thechannel length of the transistor drops below 0.5 microns. Thisundesirable result can only be partially offset by implementation of theLDD structures described with respect to FIG. 2. The presence of lightlydoped drain structures does not fully restore the BVDSS of thetransistor to a desired range. (It is generally considered desirable tohave BVDSS at least 1.5 to 3 times greater than the normal operatingvoltage of the particular technology). As discussed previously, shortchannel devices are desirable because of the larger number of suchdevices that can be fabricated within a given area. Therefore, it ishighly desirable to design and fabricate a semiconductor structure andprocess resulting in transistors having increased tolerance to BVDSS forchannel lengths well below 0.5 microns.

SUMMARY OF THE INVENTION

The problems identified above are in large part addressed by transistorsand integrated circuits fabricated according to a semiconductormanufacturing process in which the drain region is laterally displacedfrom a sidewall of the conductive gate. In alternative embodiments, thesource region may also be displaced from the lateral position of theconductive gate. Displacing the drain region and, in some cases thesource region, increases the transistor's tolerance to BVDSS and permitsthe fabrication of deep submicron channel transistor, exhibitinguncharacteristically high breakdown voltages. Specific embodiments maytake on a variety of forms as discussed in greater detail below. Inaddition, these detached drain transistors may be integrated into aconventional transistor fabrication process to produce devices andintegrated circuits including both detached drain and more conventionaltransistors. In this manner, desired devices may be selectivelyfabricated to exhibit a higher BVDSS while the remaining transistors arefabricated according to a more conventional design and process flow.

Broadly speaking, the present invention, in a first implementation,comprises a detached drain transistor. The detached drain transistorincludes a semiconductor substrate, a source impurity distribution, adrain impurity distribution, a gate dielectric, and a conductive gate.The source impurity distribution is substantially contained within asource region of the semiconductor substrate. The drain impuritydistribution is substantially contained within a detached drain regionof the semiconductor substrate. The gate dielectric is formed on anupper surface of the semiconductor substrate. The conductive gate isformed on the gate dielectric and laterally disposed over a channelregion of the semiconductor substrate. The channel region extendslaterally between the source region of the semiconductor substrate andthe detached drain region. The channel boundary of the detached drainregion is laterally displaced from a first sidewall of the conductivegate by a detached displacement.

Preferably, the semiconductor substrate comprises single crystal siliconhaving a resistivity of approximately 10 to 15 Ω-cm. In one embodiment,the source region includes a lightly doped source region and a heavilydoped source region. The lightly doped source region extends to a firstdepth below the upper surface of the semiconductor substrate and islaterally displaced within the substrate such that a channel boundary ofthe lightly doped source region is approximately coincident with alateral position of a second sidewall of a conductive gate. The heavilydoped source region extends to a second depth below the upper surface ofthe substrate. The second depth is greater than the first depth and theheavily doped source region is laterally displaced within thesemiconductor substrate such that an interior boundary of the heavilydoped source region is displaced from the second sidewall of theconductive gate by a source displacement. In this embodiment, the sourceimpurity distribution includes a first source impurity distributionsubstantially contained within the lightly doped source region and asecond source impurity distribution substantially contained within theheavily doped source region. A peak concentration of the second sourceimpurity distribution is preferably greater than a peak concentration ofthe first impurity distribution. Preferably, a peak concentration of thedrain impurity distribution is approximately equal to a peakconcentration of the second source impurity distribution and the draindisplacement is approximately equal to the source displacement.

Preferably, the gate dielectric is a thermal oxide having a thickness ofapproximately 20 to 200 angstroms. The conductive gate preferablycomprises polysilicon having a sheet resistivity of less thanapproximately 500 Ω/square. Alternatively, the conductive gate maycomprise a metal of aluminum, copper, tungsten or alloys thereof. Thelateral dimension of the conductive gate is ideally less thanapproximately 0.3 microns, while the drain displacement, in thisembodiment, is approximately 500 to 1500 angstroms.

The present invention still further contemplates a semiconductormanufacturing process. The process includes the steps of providing asemiconductor substrate, forming a gate dielectric on an upper surfaceof the semiconductor substrate, forming a conductive gate on the gatedielectric, introducing a source impurity distribution into a sourceregion of the semiconductor substrate, and introducing a drain impuritydistribution into a detached drain region. The semiconductor substrateincludes a channel region that is laterally disposed between the sourceregion and the detached drain region of the semiconductor substrate. Theconductive gate is formed such that a first sidewall of the conductivegate is laterally displaced a drain displacement from a channel boundaryof the detached drain region and further such that a lateral position ofa second sidewall of the conductive gate is approximately coincidentwith a lateral position of a channel boundary of the source region ofthe semiconductor substrate. The drain impurity distribution isintroduced into the detached drain region of the semiconductor substratewhereby the drain impurity distribution is laterally displaced from thefirst sidewall of the conductive gate by the drain displacement.

The step of forming the gate dielectric preferably comprises thermallyoxidizing the upper surface of the semiconductor substrate at a temp ofgreater than approximately 800° C. to form a thermal oxide ofapproximately 20 to 200 angstroms in thickness. The conductive gate maybe formed by depositing a conductive film on the gate dielectric andpatterning the conductive film with a photolithography masking step. Inone embodiment, the deposition of the conductive film compriseschemically vapor depositing polysilicon and introducing a gate impuritydistribution into the polysilicon to reduce the sheet resistivity of thepolysilicon to less than approximately 500 Ω/square. In an alternativeembodiment, the step of depositing the conductive film comprises sputterdepositing a metal selected from a group consisting of aluminum, copper,tungsten, and alloys thereof. The source region, in one embodiment,includes a lightly doped source region and a heavily doped sourceregion. In this embodiment, the step of introducing the source impuritydistribution into the source region comprises ion implanting a firstsource impurity distribution into the semiconductor substrate, forming aspacer structure on the second sidewall of the conductive gate, and ionimplanting a second source impurity distribution into the semiconductorsubstrate. In this embodiment, the first source impurity distribution ision implanted into the lightly doped impurity region of thesemiconductor substrate such that the lightly doped impurity regionssubstantially contains the first source impurity distribution. Thelightly doped impurity region extends to a first depth below the uppersurface of the semiconductor substrate. The spacer structure is formedon the second sidewall of the conductive gate such that the spacerstructure protects a portion of the source region proximal to the secondsidewall. The second source impurity distribution is implanted into thesemiconductor substrate such that the heavily doped impurity regionsubstantially contains the second source impurity distribution andwherein the heavily doped impurity region extends to a second depthbelow the upper surface of the semiconductor substrate. The first depthis less than the second depth and a peak concentration of the firstsource impurity distribution is less than a peak concentration of thesecond source impurity distribution. Preferably, the step of introducinga drain impurity distribution includes forming a spacer structure on thefirst sidewall of the conductive gate. Ideally, the lateral dimension ofthe spacer structure is approximately equal to the drain displacementsuch that the first spacer structure protects a portion of the channelregion laterally displaced between the channel boundary of the detacheddrain region and a lateral position of the first sidewall of theconductive gate. The drain displacement is preferably less thanapproximately 0.15 microns. Preferably, the steps of introducing theimpurity distributions into the semiconductor substrate is comprisedimplanting ions of arsenic, boron or phosphorous into the semiconductorsubstrate. In one such embodiment, an implant dose for the step ofintroducing the drain impurity distribution is in the range ofapproximately 2×10¹² to 5×10¹⁵ atoms/cm².

In a second implementation, the present invention still furthercontemplates an alternative embodiment of the detached drain transistorcomprising a semiconductor substrate, a gate dielectric, and a first anda second source/drain impurity distribution. The substrate includes achannel region laterally displaced between a first and a secondsource/drain region. The gate dielectric is formed on an upper surfaceof the semiconductor substrate. The conductive gate is formed on thegate dielectric and includes a first and a second sidewall. The firstand second sidewalls of the conductive gate are laterally displaced fromchannel boundaries of the first and second source/drain regions by asource/drain displacement. The first and second source/drain impuritydistributions are substantially contained within the first and secondsource/drain regions respectively of the semiconductor substrate.Preferably, the transistor further includes a gate insulator formed incontact with the first and second sidewalls of the semiconductorsubstrate. Respective exterior sidewalls of the gate insulator arelaterally displaced from the first and second sidewalls by a firstdisplacement. In one embodiment, the gate insulator includes an innerinsulator comprising a first dielectric material and an outer insulatorcomprising a second insulator material. In one such embodiment, theinner insulator comprises oxide and the outer insulator comprisessilicon nitride. In one embodiment, a thickness of the inner insulatoris approximately 50 to 100 angstroms and a thickness of the outerinsulator is less than or equal to approximately 200 angstroms. The gatedielectric typically comprises an oxide having a thickness ofapproximately 20 to 200 angstroms while the conductive gate may comprisepolysilicon having a sheet resistivity of less than approximately 500Ω/square or, alternatively, the conductive gate may comprise aluminum,copper, tungsten or alloys thereof. A lateral dimension of theconductive gate is preferably less than 0.3 microns and the breakdownvoltage of the transistor is greater than approximately 7 volts.Preferably, the first and second source/drain impurity distributionscomprise ions of arsenic, phosphorous or boron and a peak concentrationof the first and second source/drain impurity distributions ispreferably greater than 1×10¹⁹ atoms/cm³. In one embodiment, thesource/drain displacement is approximately 200 to 300 angstroms.

The present still further contemplates an integrated circuit. Theintegrated circuit comprises a semiconductor substrate that includes afirst transistor region and second transistor region laterally displacedfrom the first transistor region. The integrated circuit furtherincludes a first transistor formed over the first transistor region andsecond transistor formed over the second transistor region. The firsttransistor is formed according to the transistor of the previousparagraph. The second transistor includes a second conductive gate, apair of lightly doped impurity distributions, and a pair of heavilydoped impurity distributions. The pair of lightly doped impuritydistributions are substantially contained within a pair of lightly dopedimpurity regions laterally displaced on either side of a channel regionof the second transistor region. The channel boundaries of the lightlydoped regions are approximately coincident with lateral positions of thefirst and second sidewalls of the second conductive gate. A peakconcentration of the lightly doped impurity distributions is less thanapproximately 5×10¹⁷ atoms/cm³. The pair of heavily doped impuritydistributions are substantially contained within a pair of heavily dopedimpurity regions laterally displaced on either side of the channelregion of the second transistor. Interior boundaries of the heavilydoped regions are laterally displaced from the first and secondsidewalls of the second conductive gate by approximately thesource/drain displacement of the first transistor. A peak concentrationof the heavily doped impurity distributions is greater thanapproximately 1×10¹⁹ atoms/cm³.

The present invention still further contemplates a second implementationof a semiconductor process. The process includes providing asemiconductor substrate, forming a gate dielectric on the semiconductorsubstrate, forming a conductive gate, and introducing first and secondimpurity distributions into the semiconductor substrate. Thesemiconductor substrate includes a channel region laterally displacedbetween a first and second source/drain region. The conductive gate isformed on an upper surface of the gate dielectric such that first andsecond sidewalls of the conductive gate are laterally displaced fromrespective channel boundaries of the first and second source/drainregions by a source/drain displacement such that a displacement betweenthe channel boundaries is greater than a lateral dimension of theconductive gate. Thereafter, first and second impurity distributions areintroduced into the semiconductor substrate such that the first andsecond source/drain regions substantially contain the respectiveimpurity distributions. Preferably the step of forming the gatedielectric comprises thermally oxidizing the upper surface ofsemiconductor substrate at a temperature greater than approximately 800°C. for a duration sufficient such that a thickness of the gatedielectric is approximately 20 to 200 angstroms. In one embodiment, theprocess further comprises prior to the step of introducing the first andsecond impurity distributions, thermally oxidizing the conductive gateto form an inner gate insulator in contact with the first and secondsidewalls of the conductive gate and wherein the thickness of the innergate insulator is approximately 50 to 100 angstroms. Subsequently, asecond gate insulator is preferably formed on the exterior sidewalls ofthe inner gate insulator. A preferred thickness of the second gateinsulator is approximately 150 to 250 angstroms. Ideally, the secondgate insulator is formed by depositing silicon nitride upon thetopography defined by the first gate insulator and the upper surface ofthe semiconductor substrate and anisotropically etching the siliconnitride layer to remove portions of the silicon nitride substantiallyparallel to the upper surface of the semiconductor substrate whereby thesecond gate insulator comprises spacer structures in contact withexterior sidewalls of the inner gate insulator.

In a third implementation, the present invention still furthercontemplates an integrated circuit comprising a semiconductor substrate,a gate dielectric formed on an upper surface of the semiconductorsubstrate, a first and a second conductive gate formed on the gatedielectric, a lightly doped impurity distribution, and a first sourceimpurity distribution, and a detached impurity distribution. The firstand second conductive gates are formed on the gate dielectric such thatthe first conductive gate is displaced over a first transistor region ofthe semiconductor substrate and the second transistor gate is displacedover a second transistor region of the semiconductor substrate. Thelightly doped impurity distribution is introduced into the secondtransistor region such that it is substantially contained within firstand second lightly doped impurity regions laterally displaced on eitherside of a channel region of the second transistor region. A lateraldimension of the channel region of the second transistor region isapproximately equal to a lateral dimension of the second conductive gatesuch that channel boundaries of the first and second lightly dopedimpurity regions are approximately coincident with lateral positions ofthe first and second sidewalls of the second conductive gate. The firstsource impurity distribution is substantially contained within a firstsource region of the first transistor region. The first source region islaterally displaced from a channel region of the first transistorregion. A channel boundary of the first source region is approximatelycoincident with a lateral position of the second sidewall of the firstconductive gate. The detached impurity distribution is substantiallycontained within first and second pairs of detached source/drainregions. Respective pairs of the detached source/drain regions arelaterally displaced on either side of channel regions withincorresponding transistor regions. Interior boundaries of the detachedsource/drain regions are laterally displaced a source/drain displacementfrom respective sidewalls of the conductive gate. Preferably, theintegrated circuit further includes a first and a second pair of spacerstructures formed in contact with sidewalls of the first and secondconductive gates respectively. A lateral dimension of the spacerstructures is approximately equal to the source/drain displacement. Inone embodiment, a lateral dimension of the spacer structure isapproximately 500 to 1000 angstroms. Preferably, the first conductivegate has a lateral dimension less than approximately 0.3 microns. Inalternative embodiments, the first and second conductive gates maycomprise polysilicon having a sheet resistivity less than approximately500 Ω/square or the conductive gates may comprise a metal such asaluminum, copper, tungsten or alloys thereof.

The present invention still further contemplates a third implementationof an integrated circuit process comprising providing a semiconductorsubstrate, forming a gate dielectric on an upper surface of thesemiconductor substrate, forming first and second conductive gates onthe gate dielectric, introducing a lightly doped impurity distributioninto the substrate, introducing a first source impurity distributioninto the substrate, and introducing a detached impurity distributioninto the semiconductor substrate. The first and second conductive gatesare formed on the gate dielectric such that the first conductive gate isdisplaced over a first transistor region of the semiconductor substrateand the second transistor gate is displaced over a second transistorregion of the semiconductor substrates. The first transistor regionbeing laterally displaced from the second transistor region. The lightlydoped impurity distribution is introduced into the second transistorregion of the semiconductor substrate such that the lightly dopedimpurity distribution is substantially contained within first and secondlightly doped impurity regions laterally displaced on either side of achannel region of the second transistor region. A lateral dimension ofthe channel region of the second transistor region is approximatelyequal to a lateral dimension of the second conductive gate. The firstsource impurity distribution is introduced into the semiconductorsubstrate such that it is substantially contained within a first sourceregion of the first transistor region. The first source region islaterally displaced from a channel region of the first transistor regionsuch that a channel boundary of the first source region is approximatelycoincident with a second sidewall of the first conductive gate. Thedetached impurity distribution is introduced into the semiconductorsubstrate such that it is substantially contained within first andsecond pairs of detached source/drain regions. Respective pairs of thedetached source/drain regions are laterally displaced on either side ofchannel regions within corresponding transistor regions of thesemiconductor substrate. Interior boundaries of the detachedsource/drain regions are laterally displaced from respective sidewallsof respective conductive gates by a source/drain displacement.

Preferably, the step of introducing the lightly doped impuritydistribution includes implanting ions of arsenic, boron, or phosphorousat an implant energy less than approximately 20 keV and at a dose ofless than approximately 5×10¹⁴ atoms/cm². The step of introducing thefirst source impurity distribution comprises implanting ions of arsenic,boron, or phosphorous at an implant dose in the approximate range of2×10¹² to 1×10¹⁵ atoms/cm². The step of introducing the detachedimpurity distribution includes implanting ions of arsenic, boron orphosphorous using an implant dose greater than approximately 5×10¹⁴atoms/cm². In one embodiment, the process Her includes, prior tointroducing the detached impurity distribution, forming a first pair anda second pair of spacer structures on sidewalls of the first and secondconductive gates respectively. A lateral dimension of the spacerstructures is approximately equal to the source/drain displacement.

In a fourth implementation, the present invention still furthercontemplates a detached drain transistor including a semiconductorsubstrate, a gate dielectric formed on an upper surface of thesubstrate, a conductive gate formed on the gate dielectric, a first pairof spacer structures, a first source impurity distribution, a secondpair of spacer structures, and a drain impurity distribution. Theconductive gate is laterally disposed over a channel region of thesemiconductor substrate. The channel region extends laterally between afirst source region of the semiconductor substrate and a detached drainregion of the semiconductor substrate. A channel boundary of thedetached region is laterally displaced from a first sidewall of theconductive gate by a drain displacement. A channel boundary of the firstsource region is laterally displaced from a second sidewall of theconductive gate by a source displacement. The first pair of spacerstructures is formed in contact with the first and second sidewalls ofthe conductive gate. A lateral dimension of the first pair of spacerstructures is approximately equal to the source displacement. The firstsource impurity distribution is substantially contained within the firstsource region of the semiconductor substrate. The second pair of spacerstructures is formed on exterior sidewalls of the first pair of spacerstructures such that exterior sidewalls of the second pair of spacerstructures are displaced from respective sidewalls of the conductivegate by approximately said drain displacement. The drain impuritydistribution is substantially contained within the detached drain regionof the semiconductor substrate. Preferably, a lateral dimension of theconductive gate is less than approximately 0.3 microns and the breakdownvoltage of the transistor is greater than approximately 7 volts. In apresently preferred embodiment, the source displacement is approximately50 to 400 angstroms while the drain displacement is approximately 500 to1500 angstroms. In one embodiment, the first pair of spacer structurescomprises silicon nitride. The first source impurity distributionpreferably includes ions of arsenic, boron, or phosphorous and has apeak concentration in the approximate range of 1×10¹⁷ to 5×10²⁰atoms/cm³. In one embodiment, the transistor further includes a secondsource impurity distribution substantially contained within a secondsource region of the semiconductor substrate. An interior boundary ofthe second source region is laterally displaced from the second sidewallof the conductive gate by approximately said drain displacement. A peakconcentration of the second source impurity distribution isapproximately equal to a peak concentration of the drain impuritydistribution such that the second source impurity distributionrepresents a mirror image of the drain impurity distribution. In oneembodiment, the second pair of spacer structures is comprised of oxide.

In a fifth implementation, the present invention still furthercontemplates a semiconductor manufacturing process comprising providinga semiconductor substrate, forming a gate dielectric on an upper surfaceof the semiconductor substrate, forming a conductive gate on an uppersurface of the gate dielectric, forming a first pair of spacerstructures on the first and second sidewalls of the conductive gate,introducing a first source impurity distribution into the semiconductorsubstrate, forming a second pair of spacer structures on respectiveexterior sidewalls of the first pair of spacer structures, andintroducing a drain impurity distribution into the detached drain regionof the semiconductor substrate. The semiconductor substrate includes achannel region laterally displaced between a first source region and adetached drain region. The conductive gate is formed on the gatedielectric such that the conductive gate includes a first and a secondsidewall. The first pair of spacer structures is formed on the sidewallsof the conductive gate such that exterior sidewalls of the first pair ofspacer structures are displaced from the first and second sidewalls ofthe conductive gate by a source displacement. The first impuritydistribution is introduced into the first source region of thesemiconductor substrate wherein a channel boundary of the first sourceregion is laterally displaced from the second sidewall of the conductivegate by the source displacement. The first source region extends to afirst source depth below the upper surface of the semiconductorsubstrate. The formation of the second pair of spacer structures on theexterior sidewalls of the first pair of spacer structures isaccomplished such that exterior sidewalls of the second pair of spacerstructures are laterally displaced from the first and second sidewallsof the conductive gate by a drain displacement. The introduction of thedrain impurity distribution into the detached drain region of thesubstrate is accomplished such that a channel boundary of the detacheddrain region is laterally displaced from the first sidewall of theconductive gate by the drain displacement. The conductive gate maycomprise heavily doped CVD polysilicon or, alternatively, the conductivegate may be formed from a metal such as aluminum, copper, tungsten, oralloys thereof. In one embodiment, the process of forming the first pairof spacer structures includes chemically vapor depositing asubstantially conformal first dielectric layer on the topographycooperatively defined by the conductive gate and the upper surface ofthe semiconductor substrate. The conformal deposition of the firstdielectric layer in this embodiment is preferably carried out at apressure less than approximately 2 torrs. The first dielectric layer isthen anisotropically etched to remove portions of the first dielectriclayer over portions of the topography that are substantially parallelwith the upper surface of the semiconductor substrate. In one presentlypreferred embodiment, the first dielectric layer comprises siliconnitride. In one embodiment, the process of forming the second pair ofspacer structures includes chemically vapor depositing a substantiallyconformal second dielectric layer on the topography cooperativelydefined the conductive gate and the upper surface of the semiconductorsubstrate. Thereafter, the second dielectric layer is anisotropicallyetched to remove portions of the second dielectric layer above portionsof the topography that are substantially parallel with the upper surfaceof the semiconductor substrate.

The present invention still further contemplates an integrated circuitcomprising a semiconductor substrate, a gate dielectric formed on anupper surface of the semiconductor substrate, first and secondconductive gates, first and second gate insulators, a lightly dopedimpurity distribution, a first source/drain impurity distribution, and asecond source/drain impurity distribution. The semiconductor substrateincludes a first transistor region laterally displaced from a secondtransistor region. The first and second conductive gates are formed onthe gate dielectric over the first and second transistor regionsrespectively. The conductive gates each include a first and a secondsidewall. The first and second gate insulators are formed in contactwith the sidewalls of the first and second conductive gates,respectively. The lightly doped impurity distribution is substantiallycontained within lightly doped source/drain regions laterally displacedon either side of the channel region of the second transistor region.The channel boundaries of the lightly doped source/drain regions areapproximately coincident with the lateral position of the first andsecond sidewalls of the second conductive gate. The first source/drainimpurity distribution is substantially contained within firstsource/drain regions that are laterally displaced on either side of achannel region of the first transistor region. The channel boundaries ofthe first source/drain impurity distribution are laterally displacedfrom the first and second sidewalls of the first conductive gate by afirst displacement. The second source/drain impurity distribution issubstantially contained within the second source/drain regions of thefirst transistor region. The interior boundaries of the secondsource/drain regions are laterally displaced from first and secondsidewalls of the first conductive gate by a second displacement. Thesecond displacement is greater than the first displacement. Preferably,the first and second gate insulators comprise oxide and a thickness ofthe gate insulators is approximately 50 to 100 angstroms. The lightlydoped impurity distribution preferably includes ions of arsenic, boron,or phosphorous having a peak concentration less than approximately5×10¹⁷ atoms/cm³. A peak concentration of the first source/drainimpurity distribution is preferably in the range of approximately 1×10¹⁷to 5×10²⁰ atoms/cm³. A peak concentration of the second source/drainimpurity distribution is preferably greater than approximately 1×10¹⁹atoms/cm³. The first displacement is preferably in the range ofapproximately 100 to 300 angstroms. The second displacement is ideallygreater than the first displacement and the second displacement is inthe range of approximately 200 to 400 angstroms. In a preferredembodiment, the lateral dimension of the first conductive gate is lessthan approximately 0.3 microns and the breakdown voltage of the firsttransistor is greater than approximately 7 volts.

The present invention still her contemplates an integrated circuitmanufacturing process comprising providing a semiconductor substrate,forming a gate dielectric on an upper surface of the semiconductorsubstrate, forming a first and a second conductive gate on the gatedielectric, introducing a lightly doped impurity distribution into thesemiconductor substrate, forming a first and a second gate insulator,forming a first pair of dielectric spacers, introducing a firstsource/drain impurity distribution into the semiconductor substrate,removing the first pair of spacer structures, forming a second pair ofspacer structures, and introducing a second source/drain impuritydistribution into the semiconductor substrate. The semiconductorsubstrate includes a first transistor region laterally displaced from asecond transistor region. The first and second conductive gates areformed over the first and second transistor regions respectively of thesemiconductor substrate. The first and second conductive gates eachinclude a first and second sidewall. The lightly doped impuritydistribution is introduced into a first and a second lightly dopedimpurity region. The first and second lightly doped impurity regions arelaterally displaced on either side of a channel region of the secondtransistor region. The channel boundaries of the first and secondlightly doped impurity distributions are approximately coincident withlateral positions of the first and second sidewalls of the secondconductive gate. The first and second gate insulators are formed incontact with the sidewalls of the first and second conductive gatesrespectively. The first pair of dielectric spacers are formed onexterior sidewalls of the first gate insulator such that exteriorsidewalls of the first pair of spacer structures are laterally displacedfrom first and second sidewalls respectively of the conductive gate by afirst displacement distance. The first source/drain impuritydistribution is introduced into a pair of first source/drain regions.The first pair of source/drain regions are laterally displaced on eitherside of a channel region of the first transistor region. The channelboundaries of the first and second source/drain regions are laterallydisplaced from the first and second sidewalls of the first conductivegate by the first displacement distance. The second pair of spacerstructures are formed on exterior sidewalls of the first gate insulatorsuch that exterior walls of the second pair of spacer structures arelaterally displaced from the first and second sidewalls of the firstconductive gate by a second displacement distance. The secondsource/drain impurity distribution is introduced into the pair ofsource/drain regions. The pair of second source/drain regions arelaterally displaced on either side of the channel region of the firsttransistor region. Interior boundaries of the second pair ofsource/drain regions are laterally displaced from first and secondsidewalls of the first conductive gate by the second displacement. Thestep of forming the gate dielectric comprises thermally oxidizing of theupper surface of the semiconductor substrate at a temperature greaterthan approximately 800° C. for a duration sufficient to produce athickness of the gate dielectric approximately 20 to 200 angstroms. Theformation of the first and second conductive gates may comprisechemically vapor depositing polysilicon or, alternatively, sputterdepositing a metal such as aluminum, copper, or tungsten.

In one embodiment, the step of forming the first and second gateinsulators comprises thermally oxidizing exposed surfaces including thesidewalls of the first and second conductive gates. In one preferredembodiment, the step of depositing the first spacer dielectric materialcomprises chemically vapor depositing silicon nitride. The formation ofthe second pair of dielectric spacers, in one embodiment, may includedepositing a second spacer dielectric material on a topographycooperatively defined by the first and second conductive gates. Thefirst and second conductive gates, the first and second gate insulators,and the upper surface of the semiconductor substrate. The deposition ofthe second spacer dielectric material is preferably accomplished at apressure less than approximately 2 torrs such that a substantiallyconformal second spacer dielectric layer is produced. Thereafter, thesecond spacer dielectric material is anisotropically etched such thatportions of the second spacer dielectric material are removed fromplanar surfaces of the topography. For purposes of this disclosure, aplanar surface refers to a surface substantially parallel with the uppersurface of the semiconductor substrate. The deposition of the secondspacer dielectric material preferably comprises chemical vapordepositing oxide. In one embodiment, the formation of the first andsecond conductive gate includes the steps of sputter depositing a metalof aluminum, copper, or an alloy thereof onto the gate dielectric andpatterning the metal with a photolithography masking step. Ideally, thesteps of introducing the various impurity distributions into thesemiconductor substrate comprise implanting ions of arsenic, boron orphosphorous. In one such embodiment, an implant dose for the step ofintroducing the lightly doped impurity distributions into thesemiconductor substrate is less than approximately 5×10¹⁴ atoms/cm². Asuitable implant dose for the step of introducing the first source/drainimpurity distribution into the semiconductor substrate is in the rangeof approximately 2×10¹² to 5×10¹⁵ atoms/cm². A suitable implant dose forthe introduction of the second source/drain impurity distribution intothe semiconductor substrate is preferably greater than approximately5×10¹⁴ atoms/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will becomeapparent upon reading the following detailed description and uponreference to the accompanying drawings in which:

FIG. 1 is a partial cross-sectional view of a semiconductor transistor;

FIG. 2 is a partial cross-sectional view of a semiconductor transistorincluding lightly doped source/drain regions;

FIG. 3A is a partial cross-sectional view of a semiconductor substrateupon which a gate dielectric and a conductive gate have been formed;

FIG. 3B is a processing step subsequent to FIG. 3A in which a firstsource impurity distribution has been introduced into the semiconductorsubstrate;

FIG. 3C is a processing step subsequent to FIG. 3B in which spacerstructures have been formed on the sidewalls of the conductive gate;

FIG. 3D is a processing step subsequent to FIG. 3C in which an impuritydistribution has been introduced into source/drain regions displacedfrom sidewalls of the conductive gate;

FIG. 4A is a partial cross-sectional view of a first and a secondconductive gate formed over a semiconductor substrate after a lightlydoped impurity distribution has been introduced into the substrate;

FIG. 4B is a processing step subsequent to FIG. 4A in which gateinsulators have been formed in contact with the sidewalls of the firstand second conductive gate;

FIG. 4C is a processing step subsequent to FIG. 4B in which spacerstructures have been formed on the exterior sidewalls of the first andsecond gate insulator;

FIG. 4D is a processing step subsequent to FIG. 4C in which an impuritydistribution has been introduced into regions of the semiconductorsubstrate that are laterally displaced from sidewalls of the first andsecond conductive gate;

FIG. 5A is a partial cross-sectional view of a first and a secondconductive gate after a lightly doped impurity distribution has beenintroduced into the semiconductor substrate;

FIG. 5B is a processing step subsequent to FIG. 5A in which a firstsource impurity distribution has been introduced into a first sourceregion of the semiconductor substrate;

FIG. 5C is a processing step subsequent to FIG. 5B in which spacerstructures have been formed on the sidewalls of the first and secondconductive gates;

FIG. 5D is a processing step subsequent to FIG. 5C in which an impuritydistribution has been introduced into regions of the semiconductorsubstrate that are laterally displaced from sidewalls of the respectiveconductive gates;

FIG. 6A is a partial cross-sectional view of a semiconductor substrateupon which a gate dielectric and a conductive gate have been formed;

FIG. 6B is a processing step subsequent to FIG. 6A in which a first pairof spacer structures has been formed on the sidewalls of the conductivegate;

FIG. 6C is a processing step subsequent to FIG. 6B in which a firstsource impurity distribution has been introduced into a first sourceregion of the semiconductor substrate;

FIG. 6D is a processing step subsequent to FIG. 6C in which an impuritydistribution has been introduced into regions of the semiconductorsubstrate laterally displaced from sidewalls of the conductive gateafter a second pair of spacer structures has been formed on the exteriorsidewalls of the first pair of spacer structures;

FIG. 7A is a partial cross-sectional view of a first and secondconductive gate formed on a gate dielectric over a semiconductorsubstrate after a lightly doped impurity distribution has beenintroduced into the semiconductor substrate;

FIG. 7B is a processing step subsequent to FIG. 7A in which an impuritydistribution has been introduced into regions of the semiconductorsubstrate laterally displaced from the sidewalls of the first and secondconductive gate after the formation of a gate insulator and a first pairof spacer structures; and

FIG. 7C is a processing step subsequent 7B in which an impuritydistribution is introduced into regions of the semiconductor substratelaterally displaced from sidewalls of the first and second conductivegate after the formation of a second spacer structure upon the exteriorwalls of the gate insulator.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now to the drawings, five implementations of the presentinvention are disclosed and will herein be described in detail. FIGS. 3Athrough 3D disclose a first implementation of a processing sequence forforming a detached drain transistor 300 (shown in FIG. 3D). Detacheddrain transistor 300 includes semiconductor substrate 302, a sourceimpurity distribution 309, a drain impurity distribution 328a, a gatedielectric 304, and a conductive gate 306 formed on gate dielectric 304.Conductive gate 306 is laterally disposed over a channel region 311 ofsemiconductor substrate 302. Channel region 311 extends laterallybetween channel boundary 315 of source region 313 and channel boundary329 of detached drain region 326a. Channel boundary 329 of detacheddrain region 326a is laterally displaced from first sidewall 308a ofconductive gate 306 by a drain displacement d. Source impuritydistribution 309 is substantially contained within source region 313.Similarly, drain impurity distribution 328a is substantially containedwithin detached drain region 326a of semiconductor substrate 302. Itwill be appreciated to those skilled in the art of semiconductorprocessing that impurity distributions such as source impuritydistribution 309 and drain impurity distribution 328a representconcentrated populations of ions commonly found in semiconductorapplication such as phosphorous, arsenic, and boron. Because impuritydistributions found within semiconductor substrates do not have discreetboundaries, the impurity regions shown in the figures representboundaries within which greater than 90% of the respective impuritydistribution resides. It will be further appreciated that the conductivegate 306 of detached drain transistor 300 does not extend fully todetached drain region 326a. It is theorized that the displacement ofdrain region 328a away from the lateral position of conductive gate 306will result in a transistor demonstrating an improved drain breakdownvoltage characteristic. For purposes of this disclosure, the drainbreakdown voltage (BVDSS) represents the voltage that, when applied tothe drain terminal of the transistor, produces a drain current exceedinga predetermined value. BVDSS may be measured by grounding the gatesource and substrate terminals of detached drain transistor 300 whileramping the drain bias voltage and monitoring the drain current todetect the onset of current flow from drain region 326a.

Semiconductor substrate 302 preferably comprises single crystal silicon.In one embodiment, semiconductor substrate 302, prior to the processingshown in FIGS. 3A through 3D, includes a bulk region and a lightly dopedepitaxial layer formed on top of the bulk. Prior to the processing shownin the figures, a preferred resistivity of semiconductor substrate 302is in the range of approximately 10 to 15 Ω-cm. The source region 313,in a presently preferred embodiment includes two separate sourceimpurity distributions. First source impurity distribution 314 isintroduced into semiconductor substrate 302 prior to the introduction ofsecond source impurity distribution 326b. In this embodiment, firstsource impurity distribution 314 is substantially contained within firstsource region 316 while second source impurity distribution 326b issubstantially contained within second source region 328b. First sourceregion 316 extends to a first depth x₁ below the upper surface of thesemiconductor substrate while heavily doped source region 328b extendsto a second depth x₂ wherein second depth x₂ is greater than first depthx₁. Channel boundary 315 of source region 313 also represents thechannel boundary of first region 316. Interior boundary 317 of secondsource region 328b is laterally displaced from channel boundary 315 by adisplacement amount approximately equal to the drain displacement d. Inthis embodiment, source impurity distribution 309 includes first sourceimpurity distribution 314 and second source impurity distribution 326b.First source impurity distribution 314 is substantially contained withinfirst source region 316 while second source impurity distribution 326bis substantially contained within second source region 328b. In onepresently preferred embodiment, drain impurity distribution 328a andsecond source impurity distribution 326b are introduced intosemiconductor substrate 302 simultaneously such that their peakconcentrations are approximately equal.

Preferably, gate dielectric 304 is a thermal oxide having a thickness ofapproximately 20 to 200 angstroms. In one embodiment, conductive gate306 comprises polysilicon. In such an embodiment, the polysilicon istypically heavily doped with an appropriate impurity to reduce the sheetresistivity of the polysilicon to less than approximately 500 Ω/square.An appropriate impurity for n-channel transistors includes phosphorousand arsenic while an appropriate impurity for a p-channel transistorwould include boron. In an alternative embodiment, conductive gate 306is formed of a metal such as aluminum, copper, tungsten, and variousalloys thereof. As discussed previously, BVDSS is typically a concernonly in short channel devices. The preferred embodiment of the presentinvention contemplates a short channel (i.e., less than approximately0.3 microns) device in which the BVDSS exceeds approximately 7 volts. Ina first embodiment, a lateral dimension of conductive gate 306 is withina first channel length range of approximately 0.2 to 0.3 microns. In asecond embodiment, a lateral dimension of conductive gate 306 is withina second channel length range of approximately 0.1 to 0.2 microns and,in a third embodiment, a lateral dimension of conductive gate 306 iswithin a third channel length range of approximately 0.05 to 0.1microns. In one embodiment, detached drain transistor 300 includes firstand second spacer structures 320a and 320b formed respectively on firstand second sidewalls 308a and 308b of conductive gate 306. The presenceof the spacer structure facilitates the displacement of detached drainregion 326a from conductive gate 306. As will be appreciated to thoseskilled in the art, a lateral dimension of spacer structure 320 willapproximate the drain displacement d. It is understood that some lateralencroachment of drain impurity distribution 328a will occur and thatsubsequent processing at elevated temperatures may serve to laterallydisplace channel boundary 329 towards a lateral position of firstsidewall 308a. It is contemplated, however, that by minimizing thesubsequent high temperature processing to which detached draintransistor 300 is subjected, the lateral dimension of spacer structures320 should exceed the drain displacement d by not more thanapproximately 300 angstroms.

In an n-channel embodiment of detached drain transistor 300, drainimpurity distribution 328a, first source impurity distribution 314, andsecond source impurity distribution 326b all comprise a concentration ofphosphorous or arsenic ions. In a p-channel embodiment, these impuritydistributions comprise boron ions. In a presently preferred embodiment,a peak concentration of drain impurity distribution 328a and secondsource impurity distribution 326b is preferably greater thanapproximately 1×10¹⁹ atoms/cm³ while a peak concentration of firstsource impurity distribution 314 is preferably in the range ofapproximately 1×10 ¹⁷ to 5×10²⁰ atoms/cm². A peak concentration ofsecond source impurity distribution 326b is preferably greater than apeak concentration of first source impurity distribution 314. It istheorized that the lateral displacement of drain impurity distribution326a increases the channel resistance and decreases the saturated draincurrent of detached drain transistor 300, relative to a moreconventional transistor in which the drain impurity distribution is notlaterally displaced from the conductive gate. The lower saturated draincurrent is compensated for by the significantly increased breakdownvoltage and, accordingly, reliability of the transistor. The increase inBVDSS allows the circuit designer to decrease the lateral dimension ofconductive gate 306 thereby producing a smaller and more reliabletransistor. It is contemplated that the decrease in saturated draincurrent may be adequately compensated for by the decrease in effectivechannel length and that the decreased channel length will not beaccompanied by decreased reliability in the form of a lower breakdownvoltage.

A preferred processing sequence for fabrication of detached draintransistor 300 is shown in FIGS. 3A through 3D. In FIG. 3A asemiconductor substrate 302 is provided. Semiconductor substrate 302includes a channel region 311 laterally disposed between a source region313 and a detached drain region 326a. Gate dielectric 304 is then formedon an upper surface 301 of semiconductor substrate 302. Gate dielectric304 is preferably formed by thermally oxidizing semiconductor substrate302 in an oxygen bearing ambient maintained at a temperature greaterthan approximately 800° C. for a duration sufficient to form a thermaloxide film of approximately 20 to 200 angstroms in thickness. The hightemperature processing necessary to thermally oxidize semiconductorsubstrate 302 may be accomplished in a batch type system such as a tubefurnace. Alternatively, rapid thermal processing techniques may beemployed to fabricate the thermal oxide. In a rapid thermal process, asingle wafer is typically subjected to an elevated temperature for aduration comparatively brief with respect to comparable furnaceprocesses. Subsequent to the formation of thermal dielectric 304,conductive gate 306 is formed upon gate dielectric 304 and laterallydisplaced above channel region 311 of semiconductor substrate 302. Inone embodiment, the formation of conductive gate 306 includes the stepsof depositing a conductive film upon the gate dielectric and patterningthe conductive film with a photolithography masking step as is wellknown in the field of semiconductor processing, including an anisotropicetch process that produces substantially vertical first and secondsidewalls 308a and 308b respectively. In one embodiment, the depositionof the conductive film includes the steps of chemically vapor depositingpolysilicon typically at a temperature greater than approximately 500°C. and subsequently introducing a gate impurity into the polysilicon toreduce the sheet resistivity of the polysilicon to less thanapproximately 500 Ω/square. The conductivity type of the impuritypreferably matches the conductivity type of the subsequently formedsource/drain impurity distributions. As an alternative to thepolysilicon process just described, the deposition of the conductivefilm may comprise sputter depositing a metal such as aluminum, copper,tungsten, or various alloys thereof. Subsequent to the formation ofconductive gate 306, patterned photoresist layer 310 is formed to maskdetached drain region 326a and exposed portions of channel region 311thereby leaving source region 313 exposed.

Turning to FIG. 3B, first source impurity distribution 314 is introducedinto first source region 316 of semiconductor substrate 302 with ionimplantation step 312. Accordingly, a channel boundary 318 of firstsource region 316 will be approximately coincident with a lateralposition of second sidewall 308b of conductive gate 306. Preferably,implant 312 is performed using an implant energy less than approximately20 keV and using a dose in an approximate range of 1×10¹⁷ to 5×10²⁰atoms/cm².

Turning to FIG. 3C, spacer structures 320a and 320b are formed onrespective sidewalls 308a and 308b of conductive gate 306. The formationof spacer structures such as spacer structure 320a and 320b is wellknown in the field and is accomplished by depositing a substantiallyconformal dielectric film upon the topography defined by the conductivegate 306 and upper surface 301 of semiconductor substrate 302. Forpurposes of this disclosure, a substantially conformal film refers to afilm wherein a thickness of the film in horizontal regions of the filmis within 20% of the film thickness in vertical regions. Formation of aconformal dielectric layer is facilitated by low pressure (i.e., lessthan 2 torrs) chemical vapor deposition. In a preferred embodiment, theformation of spacer structures 320a and 320b is accomplished bychemically vapor depositing an oxide film from a TEOS or silane sourceat a pressure less than approximately 2 torrs. Thereafter, the oxidefilm is anisotropically etched to clear horizontal portions of the film.The lateral dimensions of the spacer structure so formed will directlyinfluence the lateral displacement of the detached drain region. In FIG.3D, a source/drain impurity distribution (328a and 326b) is introducedinto drain region 326a and second source/drain region 326b. Implant 324preferably is performed with an implant dose greater than approximately5×10¹⁴ atoms/cm².

FIGS. 4A through 4D depict a second implementation of the presentinvention. FIG. 4D shows an integrated circuit 400 comprised of a firsttransistor 401 and a second transistor 405 formed within a firsttransistor region 410 and a second transistor region 412 respectively ofa semiconductor substrate 402. The first transistor 401 includes a gatedielectric 404 formed on upper surface of semiconductor substrate 402, aconductive gate 406a formed on the gate dielectric 404, and asource/drain impurity distribution 436 substantially contained within apair of detached source/drain regions 438a and 438b of semiconductorsubstrate 402. Conductive gate 406a of first transistor 401 includes afirst and a second sidewall 408a and 408b. First and second sidewalls408a and 408b are laterally displaced from respective channel boundaries440a and 440b of the pair of detached source/drain regions 438a and 438bby a source/drain displacement d₃. Accordingly, a lateral dimension ofconductive gates 406a is less than a lateral dimension between channelboundaries 440a and 440b of detached source/drain regions 438a and 438brespectively. Like the first implementation of the present inventiondisclosed with respect to FIGS. 3A through 3D, a suitable startingmaterial for semiconductor substrate 402 is a single crystal siliconhaving a resistivity in the range of approximately 10-15Ω-cm. Gatedielectric 404 is preferably an oxide having a thickness ofapproximately 20-200 angstroms and conductive gates 406a and 406b maycomprise polysilicon suitably doped to reduce the sheet resistivity ofthe polysilicon to less than approximately 500 Ω/square. Alternatively,conductive gate 406a and 406b may comprise a metal such as aluminum,copper, tungsten, and suitable alloys. Integrated circuit 400 furtherincludes second transistor 405. Second transistor 405 includes a secondconductive gate 406b formed on gate dielectric 404 over secondtransistor region 412 of semiconductor substrate 402, a lightly dopedimpurity distribution 422 substantially contained within a pair oflightly doped impurity regions 420a and 420b laterally displaced oneither side of a channel region 418 of second transistor region 412, anda heavily doped impurity distribution 436 substantially contained withina pair of heavily doped impurity regions 438c and 438d laterallydisplaced on either side of channel region 418. Channel boundaries ofthe pair of lightly doped impurity regions 420a and 420b areapproximately coincident with lateral positions of respective sidewalls408c and 408d of second conductive gate 406b. Interior boundaries 439aand 439b of the heavily doped regions 438c and 438d respectively aredisplaced from first and second sidewalls 408c and 408d of secondconductive gate 406b by approximately the source/drain displacement d₃.A peak concentration of the lightly doped impurity distributions 422 isless than approximately 5×10¹⁷ atoms/cm² while a peak concentration ofthe heavily doped impurity distribution 436 is greater thanapproximately 1×10¹⁹ atoms/cm³.

In a preferred embodiment, first transistor 401 further includes a gateinsulator 433 formed in contact with first and second sidewalls 408a and408b of conductive gate 406a. Exterior side walls of gate insulator 433are laterally displaced from respective sidewalls of first conductivegate 406a by approximately the source/drain displacement d₃. In oneembodiment, gate insulator 433 includes an inner insulator 430 comprisedof a first dielectric material and an outer insulator 432 comprised of asecond insulator material. Inner insulator 430, in one embodiment, iscomprised of an oxide material while outer insulator 432 is comprised ofsilicon nitride. A preferred thickness of the inner insulator isapproximately 50 to 100 angstroms while a preferred thickness of theouter insulator is less than or equal to approximately 200 angstroms. Inthe presently preferred embodiment, a lateral dimension of firstconductive gate 406a is less than approximately 0.3 microns andbreakdown voltage of first transistor 401 is greater than approximately7 volts. A lateral dimension of first conductive gate 406a is in therange of approximately 0.2 to 0.3 microns in a first embodiment, in therange of approximately 0.1 to 0.2 in a second embodiment, and in therange of approximately 0.05 to 0.1 microns in a third embodiment. Thesource/drain displacement d₃ is preferably in the approximate range of200 to 300 angstroms. The present invention still further contemplates asemiconductor manufacturing process performing a first transistor 401.The process includes the steps of providing semiconductor substrate 402,forming gate dielectric 404 upon an upper surface of semiconductorsubstrate 402, forming first conductive gate 406a on an upper surface ofgate dielectric 404, and introducing a source/drain impuritydistribution 436 into a pair of detached source/drain regions 438a and438b of semiconductor substrate 402. The pair of detached source/drainregions 438a and 438b are laterally displaced on either side of achannel region 414 within first transistor region 410 of thesemiconductor substrate 402. Conductive gate 406a is formed such thatfirst and second sidewalls 408a and 408b respectively are laterallydisplaced from respective channel boundaries 440a and 440b of the pairof detached source/drain regions 438a and 43b such that a displacementbetween the channel boundaries 440a and 440b is greater than a lateraldimension of first conductive gate 406a. Similar to the process forforming the transistor of the first implementation of the presentinvention, the step of forming gate dielectric 404 preferably comprisesthermally oxidizing an upper surface of semiconductor substrate 402 at atemperature greater than approximately 800° C. for a duration sufficientto fabricate a gate dielectric of approximately 20-200 angstroms inthickness. The preferred process forms first conductive gate 406a andsecond conductive gate 406b, in one embodiment, by chemically vapordepositing polysilicon, introducing a gate impurity distribution intothe polysilicon, and patterning the polysilicon with a photolithographymasking step. Alternatively, a metal gate process may be suitablyemployed in which conductive gates 406a and 406b are formed by sputterdepositing and metal such as aluminum, copper, tungsten, or alloysthereof onto the gate dielectric 404, and patterning the metal with aphotolithography masking step. In a preferred embodiment, the processfurther includes, prior to the step of introducing the source/drainimpurity distribution, thermally oxidizing conductive gates 406a and406b to form an inner gate insulator 430 in contact with sidewalls 408of first and second conductive gates 406a and 406b. A preferredthickness of inner gate insulator 430 is approximately 50 to 100angstroms. Thereafter, an outer gate insulator 432 is formed uponexterior sidewalls of inner gate insulator 430. A preferred thickness ofouter gate insulator 432 is approximately 150 to 250 angstroms. In onepresently preferred embodiment, outer gate insulator 432 is formed bydepositing silicon nitride upon a topography cooperatively defined bythe first gate insulator and the upper surface of the semiconductorsubstrate, and, thereafter, isotropically etching the silicon nitridelayer to remove portions of the silicon nitride layers that aresubstantially parallel to the upper surface of the semiconductorsubstrate. In this manner, the outer gate insulator 432 comprises a setof space or structures in contact with exterior sidewalls of inner gateinsulator 430.

Lightly doped impurity distribution 422 is introduced into the pair oflightly doped source/drain regions 420 and 420b within second transistorregion 412 of semiconductor substrate 402 preferably through the use ofan ion implantation 421. Prior to performing implantation 421, aphotoresist mask 409 is patterned to protect first transistor region 410from ion implantation 421. With second conductive gate 406b serving as amask within second region 412, it would be appreciated that channelboundaries 423a and 423b of lightly doped source/drain regions 420a and420b respectively are approximately coincident with lateral positions ofsidewalls 408c and 408d of second conductive gate 406b. In one preferredembodiment, ion implantation 421 is carried out at an implant energyless than approximately 20 keV using a dose of less than approximately5×10¹⁴ atoms/cm². FIG. 4D discloses that, in one embodiment, theimpurity distribution within the pair of detached source/drain regions438a and 438b and the impurity distribution within the pair of heavilydoped source/drain regions 438c and 438d of second transistor region 412may be simultaneously introduced into their respective regions with asingle implantation 434. In a preferred embodiment, this implantation ispreferably accomplished using an implant dose of greater thanapproximately 5×10¹⁴ atoms/cm². FIGS. 4B and 4C disclose that theformation of inner insulator 430 and outer insulator 432 may occur, inone embodiment, simultaneously upon first conductive gate 406a andsecond conductive gate 406b. In such an embodiment, implant 434 shown inFIG. 4D results in the formation of impurity distributions that arelaterally displaced from respective sidewalls of the respectiveconductive gates. More specifically, the lateral displacement betweenchannel boundaries 440 and conductive gate sidewalls 408 within firsttransistor region 410 is approximately equal to the lateral displacementbetween interior boundaries 439 and sidewalls 408 within secondtransistor region 412. It would be appreciated to those skilled in theart that second transistor 405 resembles a conventionally formed LDDtransistor in which lightly doped source/drain regions extend tosidewalls of the conductive gate while more heavily doped source/drainregions are laterally displaced from the sidewalls of the conductivegate. Thus, this implementation results in a process designed tofabricate detached source/drain transistors 401 and conventional LDDtransistors 405. In a CMOS embodiment, patterned masking layer 409 maybe incorporated into an existing LDD implant mask such that thedisclosed process requires no additional masking steps.

FIGS. 5A-5D disclose a third implementation of the present invention.FIG. 5D depicts integrated circuit 500 which includes a semiconductorsubstrate 502, a gate dielectric 504 formed on the semiconductorsubstrate 504, a first and a second conductive gate 506a and 506b formedon gate dielectric 504 over a first transistor region 516 and alaterally displaced second transistor region 518 respectively, a lightlydoped impurity distribution 512 contained within lightly dopedsource/drain regions 514a and 514b laterally displaced on either side ofa channel region 515 within second transistor region 518, a first sourceimpurity distribution 524 substantially contained within a first sourceregion 526 of first transistor region 516 and a detached impuritydistribution 540 substantially contained within first and second pairsof detached source/drain regions (shown in the figure with referencenumerals 542a, 542b, 544a, and 544b). First conductive gate 506aincludes a pair of sidewalls 508a and 508b while second conductive gate506b includes a pair of sidewalls 508c and 508d. Channel boundaries oflightly doped impurity regions 514a are substantially coincident withlateral positions of first and second sidewalls 508c and 508d of secondconductive gate 506b. A peak concentration of lightly doped impuritydistribution 512 is less than approximately 5×10¹⁷ atoms/cm³. Firstsource region 526 is laterally displaced with respect to channel region528 of first transistor region 516. A channel boundary 530 of firstsource region 526 is laterally coincident with the second sidewall 508bof first conductive gate 506a. A peak concentration of first sourceimpurity distribution 524 is in the approximate range of 1×10¹⁷ to5×10²⁰ atoms/cm³. The first pair of detached source/drain regions 542aand 542b are laterally displaced on either side of channel region 528 offirst transistor region 516 such that respective interior boundaries ofdetached source/drain regions 542a and 542b are laterally displaced fromrespective sidewalls of first conductive gate 506a by a source/draindisplacement d₂. The second pair of detached source/drain regions 544aand 544b are similarly displaced such that interior boundaries of thesecond pair of detached source/drain regions 544a and 544b are displacedfrom respective sidewalls of second conductive gate 506b by thesource/drain displacement d₂.

Similar to the previous two implementations, a preferred startingmaterial semiconductor substrate 502 comprises single crystal siliconhaving a resistivity of approximately 10 to 15 Ω-cm. A preferred gatedielectric 504 comprises a thermal oxide having a thickness ofapproximately 20 to 200 angstroms. In a preferred embodiment, integratedcircuit 500 further includes a first pair of spacer structures 534a and534b and a second pair of spacer structures 534c and 534d in contactwith respective pairs of sidewalls of the first conductive gate 506a andsecond conductive gate 506b. Each of the spacer structures has athickness approximately equal to the source/drain displacement d₂. Inone presently preferred embodiment, the spacer structure thickness isapproximately 500 to 1000 angstroms. A lateral dimension of firstconductive gate 506a is preferably less than approximately 0.3 microns.A lateral dimension of first conductive gate 506a is in the range ofapproximately 0.2 to 0.3 microns in a first embodiment, in the range ofapproximately 0.1 to 0.2 in a second embodiment, and in the range ofapproximately 0.05 to 0.1 microns in a third embodiment. First andsecond conductive gates 506a and 506b comprise heavily doped polysilicon(i.e., polysilicon having a sheet resistivity less than approximately500 Ω/square). Alternatively, first and second conductive gates 506a and506b may comprise a metal such as aluminum, copper, tungsten, or asuitable alloy.

One skilled in the art of semiconductor processing will recognize inFIG. 5D that the present implementation includes a first transistor 511and second transistor 513. First transistor 511 is of a type similar tothe detached drain transistor disclosed with respect to FIGS. 3A-3Dwhile second transistor 513 comprises an LDD type transistor. Theintegration of the detached drain transistors with the more conventionaltransistors enables the present invention to incorporate the improvedBVDSS characteristics of the detached drain transistor 511 into selectedtransistors while maintaining higher drive currents associated withsecond transistor 513 in selected transistors. The integration of thesetwo types of transistors is accomplished with a single additionalmasking step (or possibly two additional masking steps in a CMOSprocess).

Turning back now to FIG. 5A, the process performing integrated circuit500 will be described. In FIG. 5A, first and second conductive gates506a and 506b have been formed on a gate dielectric 504 over asemiconductor substrate 502. The formation of gate dielectric 504preferably comprises thermally oxidizing semiconductor substrate 502 aspreviously described with respect to the first and secondimplementations of the present invention. As in the first twoimplementations, first and second conductive gates 506a and 506b may beformed of polysilicon, or, alternatively, a metal such as aluminum,copper, tungsten, or a suitable alloy thereof. Subsequent to theformation of first and second conductive gates 506a and 506b, apatterned masking layer 509 is formed to protect first transistor region516 from a subsequent implant. Thereafter, lightly doped source/drainimpurity distribution 512 is introduced into a pair of lightly dopedsource/drain regions 514a and 514b laterally disposed on either side ofa channel region of semiconductor substrate 502. The presence of secondconductive gate 506b results in an approximate alignment between thechannel boundaries of source/drain regions 514a and 514b and respectivesidewalls 508c and 508d of second conductive gate 506b. Implant 510 ispreferably accomplished using an implant energy less than approximately20 keV and an implant dose less than approximately 5×10¹⁴ atoms/cm². Itwould be appreciated to those skilled in the art, that, in a CMOSembodiment of the present invention, pattern masking layer 509 may beintegrated with an existing LDD mask such that pattern masking layer 509does not represent an additional masking step. More specifically, in aCMOS embodiment in which a P-type lightly doped impurity distribution isintroduced into P-type regions of the semiconductor substrate while anN-type impurity distribution is introduced into N-channel regions of thesemiconductor substrate, a pattern masking layer is already necessary toprevent the introduction of the N-type lightly doped impuritydistribution into the P-type region of the semiconductor substrate, and,conversely, a pattern masking layer is generally required to prevent theintroduction of the P-type lightly doped impurity distribution into theN-type transistor regions of the semiconductor substrate.

Turning to FIG. 5B, a second pattern masking layer 520 is formed toprotect second transistor region 518 and portions of first transistorregion 516. More specifically, pattern masking layer 520 is designed toprevent the introduction of a subsequent implant into either the channelregion 528 or the detached drain region 542a (shown in FIG. 5D) ofsemiconductor substrate 502. Subsequent to the formation of patternmasking layer 520, ion implantation 522 is formed to introduce firstsource impurity distribution 524 into first source region 526 of firsttransistor region 516. As would be appreciated to those skilled in theart, a channel boundary 530 of first source region 526 will be laterallyaligned with second sidewall 508b of first conductive gate 506asubsequent to the second implant 522. In a presently preferredembodiment, implant 522 is preferably accomplished using a dose in therange of approximately 2×10¹² to 5×10¹⁵ atoms/cm².

Turning now to FIG. 5C a preferred processing step is shown in whichspacer structures 534 are formed in contact with sidewalls 508 of theconductive gates 506. The spacer structures have a thickness d₂ toapproximately equal to the source/drain displacement. Spacer structures534 facilitate the displacement of subsequently formed impuritydistribution within semiconductor substrate 502. The formation of spacerstructures 534 are preferably accomplished, in this implementation, bychemically vapor depositing a dielectric such as an oxide formed from aTEOS or silane source. The deposition is preferably performed at apressure less than approximately 2 torrs to increase the conformality ofthe deposited film. Subsequent to the oxide deposition, an anisotropicetch process is performed to remove the horizontal portions of thedielectric film leaving behind portions of the film only on verticalportions of the topography. In one presently preferred embodiment, thethickness d₂ of spacer structures 534 is approximately 500 to 1000angstroms.

FIG. 5D shows the introduction of a detached source/drain impuritydistribution 540 into detached source/drain regions 542a and 542b ofsemiconductor substrate 502 within first transistor region 516. In thepreferred embodiment, detached impurity distribution 540 is introducedinto detached source/drain regions 542 through the use of an ionimplantation step 538. The presence of spacer structures 534 results inthe displacement of detached impurity distribution 540 from sidewalls508 of conductive gate 506a where the displacement is approximatelyequal to the source/drain displacement d₂. In the preferred embodiment,ion implantation 538 is preferably carried out at an implant dosegreater than approximately 5×10¹⁴ atoms/cm². In one presently preferredembodiment designed for use in manufacturing integrated circuits thatemploy both detached drain transistors such as transistor 511 andconventionally formed transistors such as transistor 513, Ionimplantation 538 simultaneously introduces a heavily doped source/drainimpurity distribution into heavily doped source/drain regions 544a and544b within second transistor region 518. This integration of implantsteps results in a process capable of manufacturing at least two typesof transistors (i.e., the detached drain type transistor and the moreconventional LDD transistor) with only single additional masking step.It will be appreciated to those skilled in the art that masking layer520 shown in FIG. 5d may be altered such that all of second transistorregion 518 is protected during ion implantation 522 thus resulting in astandard LDD type transistor.

FIGS. 6a through 6d show a fourth implementation of the presentinvention. Turning to FIG. 6d, a detached drain transistor 600 is shown.Detached drain transistor 600 includes semiconductor substrate 602, gatedielectric 604 formed on an upper surface of semiconductor substrate602, a conductive gate 606, a first pair of spacer structures 614a and614b, a first source impurity distribution 620, a second pair of spacerstructures 624a and 624b, and a drain impurity distribution 628.Conductive gate 606 is laterally disposed over a channel region 610 ofsemiconductor substrate 602. Channel region 610 extends laterallybetween a first source region 622 (shown in FIG. 6C) and a detacheddrain region 630a (shown in FIG. 6D). The channel boundary 611 ofdetached drain region 630a is displaced from first sidewall 607a ofconductive gate 606 by a drain displacement d₂. A channel boundary 609of first source region 622 is laterally displaced from second sidewall607b of conductive gate 606 by a source displacement d₁. First pair ofspacer structures 614a and 614b are formed in contact with first andsecond sidewalls 607a and 607b respectively of conductive gate 606. Athickness of the first pair of spacer structures is approximately equalto source displacement d₁. First source impurity distribution 620 issubstantially contained within first source region 622 of semiconductorsubstrate 602. Second pair of spacer structures 624a and 624b are formedon exterior sidewalls of first pair of spacer structures 614a and 614brespectively. Exterior sidewalls 625a and 625b of second pair of spacerstructures 624a and 624b are displaced from respective sidewalls 607aand 607b of conductive gate 606 by approximately the drain displacementd₂. Drain impurity distribution 628 is substantially contained withindetached drain region 630a. Semiconductor substrate 602 like thesubstrates disclosed in the first three implementations of the presentinvention is preferably comprised of a single crystal silicon having aresistivity of approximately 10 to 15 Ωcm. The gate dielectric 604 ispreferably a thermally formed oxide having a thickness of approximately20 to 200 angstroms. Conductive gate 606 comprises either polysiliconhaving a sheet resistivity less than approximately 500 Ω/square or ametal of aluminum, copper, tungsten or an alloy thereof. A lateraldimension of conductive gate 606 is preferably less than approximately0.3 microns and a breakdown voltage of the transistor is preferablygreater than 7 volts. A lateral dimension of conductive gate 606 is inthe range of approximately 0.2 to 0.3 microns in a first embodiment, inthe range of approximately 0.1 to 0.2 in a second embodiment, and in therange of approximately 0.05 to 0.1 microns in a third embodiment. Sourcedisplacement d₁ is approximately 50 to 400 angstroms in a presentlypreferred embodiment while, in one presently preferred embodiment firstpair of spacer structures 614a and 614b are comprised of siliconnitride. First source impurity distribution 620 preferably includes ionsof arsenic, boron, or phosphorous and has a peak concentration ofapproximately 1×10¹⁷ to 5×10²⁰ atoms/cm³. Drain impurity distribution628 preferably includes ions of arsenic, boron, or phosphorous and has apeak concentration greater than approximately 1×10¹⁹ atoms/cm³. In oneembodiment detached drain transistor 600 further includes a secondsource impurity distribution 629 substantially contained within a secondsource region 630b of semiconductor substrate 602. And interior boundary631b of second source region 630b is laterally displaced from secondsidewall 607b of conductive gate 606 by approximately the draindisplacement d₂. In this embodiment, a peak concentration of the secondsource impurity distribution 629 is approximately equal to a peakconcentration of the drain impurity distribution 628 such that secondsource impurity distribution 629 substantially mirrors detached drainimpurity distribution 628.

Turing now to FIGS. 6A through 6D, a processing sequence is shown forforming detached drain transistor 600. In FIG. 6A, a conductive gate 606is formed on a gate dielectric 604 over a channel region 610 ofsemiconductor substrate 602. Channel region 610 extends laterallybetween a channel boundary 611 of a detached drain region 630a (shown inFIG. 6D) and a channel boundary 609 of first source region 622 (shown inFIG. 6C). Gate dielectric 604 is formed on an upper surface ofsemiconductor substrate 602 with a thermal oxidation process such as arapid thermal anneal or a tube furnace process as discussed previouslywith respect to FIGS. 3A through 3D. The thermal oxidation process ispreferably continued for a duration sufficient to produce a thermaloxide 604 having a thickness of approximately 20 to 200 angstroms.Thereafter, conductive gate 606 is formed by depositing a conductivematerial upon gate dielectric 604 and patterning the conductive materialwith a photolithography/etch step as is well known in the field. Thedeposition of the conductive material in one embodiment includes thechemical vapor deposition of polysilicon followed by the introduction ofan impurity distribution into the polysilicon to reduce the sheetresistivity of the polysilicon to less than approximately 500 Ω/square.Alternatively, conductive gate 606 may be formed by depositing a metalwith a sputter deposition process. Suitable metals include copper,aluminum, and tungsten. Alloys of these elements may also be suitablyemployed. In FIG. 6B, a first pair of spacer structures 614 is formed onsidewall 607a and 607b of conductive gate 606. In the presentlypreferred embodiment, first pair of spacer structures 614 are fabricatedby chemically vapor depositing silicon nitride at a pressure less thanapproximately 2 torrs to form a conformal silicon nitride layer on thetopography defined by conductive gate 606 and upper surface 601 ofsemiconductor substrate 602. Subsequent to the deposition, a spacer etchprocess is performed to produce spacer structure 614a and 614b. Thespacer etch, as will be appreciated to those skilled in the art ofsemiconductor etch technology, is typically an anisotropic etch processthat is executed with minimum overetch such that portions of the etchedfilm that are parallel to the wafer surface are removed whereas verticalportions of the film are left behind. For purposes of this disclosure, aminimum overetch process is an etch process in which the overetch cycleis less than approximately 10% of the main etch cycle. In the presentlypreferred embodiment, a thickness of the first pair of spacer structure614a and 614b is approximately 50 to 400 angstroms. The first pair ofspacer structures 614a and 614b are designed to displace the sourceregion of the transistor a small lateral displacement from the sidewallof the conductive gate.

Turning to FIG. 6c, a first source impurity distribution 620 isintroduced into a first source region 622 of semiconductor substrate 602through the use of an ion implantation process 618. Prior to theimplantation, a photoresist mask 616 is patterned to protect exposedportions of channel region 610 and detached drain region 630a ofsemiconductor substrate 602. Preferably, ion implantation 618 isperformed using an implant energy less than approximately 20 keV at animplant dose less than approximately 5×10¹⁴ atoms/cm². Turning to FIG.6D, detached drain transistor 600 is completed with the introduction ofdetached drain impurity distribution 628 into detached drain region 630aof semiconductor substrate 602. In the preferred embodiment, an ionimplantation step 626 is used to introduce detached drain impuritydistribution 628 into semiconductor substrate 602 and, simultaneously,to introduce second source impurity distribution 629 into second sourceregion 630b. Prior to performing ion implantation 626, a second pair ofspacer structures 624a and 624b are formed upon exterior sidewalls offirst pair of spacer structures 614a and 614b respectively. Second pairof spacer structures 624a and 624b are fabricated such that exteriorsidewalls 625a and 625b of the second pair of spacer structures isdisplaced from respective sidewalls 607a and 607b of conductive gate 606by a drain displacement d₂. The formation of second pair of spacerstructures 624a and 624b, in a preferred embodiment, is accomplished bychemically vapor depositing an oxide preferably formed from a TEOS orsilane source and following the deposition with a spacer etch process asdescribed above with respect to FIG. 6b.

FIGS. 7A through 7C depict a fifth implementation of the presentinvention. In FIG. 7C integrated circuit 700 is shown as including asemiconductor substrate 702, a gate dielectric 704 formed on an uppersurface of semiconductor substrate 702, first and second conductivegates 706a and 706b respectively formed on an upper surface of gatedielectric 704, first and second gate insulators 722a and 722b formed incontact with sidewalls of the first and second conductive gates 706a and706b, a lightly doped impurity distribution 714a and 714b, a firstsource/drain impurity distribution 728, and a second source/drainimpurity distribution 738. Semiconductor substrate 702 includes a firsttransistor region 708a and a laterally displaced second transistorregion 708b. First and second conductive gates 706a and 706b are formedover first and second transistor regions 708a and 708b respectively.Each conductive gate 706a includes a first sidewall (710a and 710crespectively) and a second sidewall (710b and 710d). Lightly dopedimpurity distribution 714a and 714b are substantially contained withinlightly doped impurity regions 716a and 716b of second transistor region708b. Channel boundaries 720a and 720b of lightly doped source/drainregions 716a and 716b are approximately coincident with respectivelateral positions of first and second sidewalls 710c and 710d of secondconductive gate 706b. First source/drain impurity distribution 728 issubstantially contained within first source/drain regions 730a and 730b.First source/drain regions 730a and 730b are laterally displaced oneither side of a channel region 731 of first transistor region 708a.Channel boundaries 729a and 729b of first source/drain impurity regions730a and 730b are laterally displaced from first and second sidewalls710a and 710b of first conductive gate 706a by displacement d₂. Secondsource/drain impurity distribution 738 is substantially contained withinsecond source/drain regions 740a and 740b of first transistor region708a. Interior boundaries 741a and 741b are laterally displaced fromfirst and second sidewalls 710a and 710b of first conductive gate 706aby displacement d₃. Displacement d₃ is greater than first displacementd₂. Preferably, semiconductor substrate 702 comprises silicon having aresistivity of approximately 10 to 15 Ωcm, the gate dielectric 704 ispreferably comprised of a thermal oxide and has thickness ofapproximately 20 to 200 angstroms, and the first and second conductivegates 706a and 706b may, alternatively, comprise heavily dopedpolysilicon or a metal such as aluminum, copper, or tungsten. Suitablealloys of these metals may be used as conductive gate 706. In apresently preferred embodiment, first and second insulators 722a and722b comprise oxide having a thickness of approximately 50 to 100angstroms. Lightly doped impurity distributions 714a and 714b, in apresently preferred embodiment, comprise ions of arsenic, boron, orphosphorous and have a peak concentration of less than approximately5×10¹⁷ atoms/cm³. First source/drain impurity distribution 728preferably comprises arsenic, boron or phosphorous ions and has a peakconcentration of approximately 1×10¹⁷ to 5×10²⁰ atoms/cm³. Secondsource/drain impurity distribution 738 also preferably comprises ions ofarsenic, boron, or phosphorous. A peak concentration of secondsource/drain impurity distribution 738 is preferably greater thanapproximately 1×10¹⁹ atoms/cm³.

Displacement d₂ is approximately 100 to 300 angstroms in a presentlypreferred embodiment while displacement d₃ is approximately 400angstroms. A lateral dimension of first conductive gate 706a is ideallyless than approximately 0.3 microns. In the preferred embodiment, abreakdown voltage necessary to conduct current from second source/drainregion 730b with conductive gate 706a, first source/drain region 730a,and semiconductor substrate 702 grounded is greater than approximately 7volts. A lateral dimension of first conductive gate 706a is in the rangeof approximately 0.2 to 0.3 microns in a first embodiment, in the rangeof approximately 0.1 to 0.2 in a second embodiment, and in the range ofapproximately 0.05 to 0.1 microns in a third embodiment.

Turning now to FIG. 7A, a processing sequence will be described forforming integrated circuit 700. In FIG. 7A, first and second conductivegates 706a and 706b have been formed on gate dielectric 704. Theformation of first and second conductive gate 706a and 706b ispreferably accomplished with a chemical vapor deposition of polysiliconor a sputter deposition of metal as described previously with respect tothe conductive gates of the other implementations disclosed herein.Subsequent to the formation of first and second conductive gate 706a and706b, a patterned mask 711 is formed to protect the portion ofsemiconductor substrate 702 within first transistor regions 708a.Thereafter, lightly doped impurity distribution 714 is introduced intolightly doped impurity region 716 of semiconductor substrate 702 throughthe use of an ion implantation step 712. In the preferred embodiment, animplant energy suitable for the introduction of lightly doped impuritydistribution 714 into semiconductor substrate 702 is less thanapproximately 20 keV while an implant dose is typically less thanapproximately 5×10¹⁴ atoms/cm². As will be appreciated to those skilledin the art of semiconductor processing, implant, steps such as implant712 of FIG. 7A are suitably accomplished using phosphorous, arsenic, orboron depending upon the conductivity type of the subsequently formedtransistor.

Turning now to FIG. 7B, gate insulators 722a and 722b are formedrespectively in contact with sidewall 710 of first conductive gate 706aand second conductive gate 706b. In embodiments in which first andsecond conductive gate 706a and 706b comprise polysilicon, first gateinsulator 722 is preferably formed through the thermal oxidation of thepolysilicon within conductive gate 706. Alternatively, a chemical vapordeposition process may be used to deposit first gate insulator 722. Apreferred thickness d_(t) of gate insulator 722 is preferably betweenapproximately 50 to 100 angstroms. Subsequent to the formation of gateinsulators 722a and 722b, a masking step is performed to produce apatterned mask 724 over second transistor region 708b of semiconductorsubstrate 702. Thereafter, a first pair of spacer structures 726a and726b are formed on exterior sidewalls of first gate insulator 722a. Theformation of first spacer structures 726a and 726b is preferablyaccomplished with the use of a low pressure chemical vapor depositionprocess followed by an anisotropic etch with a minimal overetch all asare well known in the art and as previously discussed herein. In apreferred embodiment, first pair of spacer structures 726a and 726bcomprise silicon nitride. The lateral dimension of first pair of spacerstructures 726a and 726 is adjusted such that exterior sidewalls offirst spacer structure 726a and 726b are displaced from respectivesidewalls of conductive gate 706a by displacement d₂. In a presentlypreferred embodiment, displacement d₂ is approximately 100 to 300angstroms. After the formation of the first pair of spacer structures726, a first source/drain impurity distribution 728 is introduced intofirst source/drain regions 730a and 730b of semiconductor substrate 702through the use of an ion implantation step 727. Due to the presence offirst pair of spacer structures 726 and first gate insulators 722,respective channel boundaries 729 of first source/drain region 730 arelaterally displaced from respective sidewalls 710 of first conductivegate 706 by approximately the displacement d₂. In the preferredembodiment, implantation 727 is performed using an implant dose in therange of approximately 2×10¹² to 5×10¹⁵ atoms/cm². Subsequent to theformation of first source/drain regions 730 within semiconductorsubstrate 702, pattern masking layer 724 and first pair of spacerstructures 726 are removed. Removal of patterned masking layer 724 ispreferably accomplished with a conventional photoresist strip while theremoval of first pair of spacer structures 726 may be accomplished witha wet etch solution selective to the dielectric used for first insulator722a. Preferably, gate insulator 722 comprises oxide and first pair ofspacer structures 726 comprise silicon nitride. In such an embodiment,the removal of spacer structures 726 can be suitably accomplished withan 85% phosphoric solution heated to approximately 120° C. Subsequent tothe removal of spacer structure 726 and patterned masking layer 724, asecond set of spacer structures 734 is formed upon exterior sidewalls offirst and second gate insulators 722a and 722b. Second pair of spacerstructures 734 are preferably formed with a chemical vapor depositionprocess using a TEOS or silane source followed by an anisotropic etchstep. A lateral dimension of second pair of spacer structures 734 isadjusted such that exterior sidewalls of spacer structures 734 arelaterally displaced from respective sidewalls of conductive gate 706 bydisplacement d₃. In the preferred embodiment, displacement d₃ isapproximately 200 to 400 angstroms. Thereafter, second source/drainimpurity distributions 738 is introduced into second source/drainregions 740a and 740b of semiconductor substrate 702 through the use ofion implantation step 736. This formation process results in a secondsource/drain impurity region having an interior boundary 741 laterallydisplaced from a respective sidewall of conductive gate 706 by thedisplacement d₃. As seen in FIG. 7c, implantation 736, in a preferredembodiment, simultaneously introduces a heavily doped source/drainimpurity distribution into the detached type transistor formed in afirst transistor region 708a and the more conventional LDD typetransistor formed in transistor region 708b. This processing sequencetherefore is compatible with existing processing sequences and requiresthe addition of just a single masking step to produce the detached draintype transistors formed in first transistor region 708a.

As will be appreciated to those skilled in the art of semiconductorfabrication, the present invention is useful for fabricating transistorsexhibiting increased BVDSS at shorter channel lengths. Furthermore, itis to be understood that the form of the invention shown and describedis to be taken as presently preferred embodiments. Various modificationsand changes may be made to each and every processing step, as would beobvious to a person skilled in the art having the benefit of thisdisclosure. It is intended that the following claims be interpreted toembrace all such modifications and changes and, accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

What is claimed is:
 1. A detached drain transistor, comprising:asemiconductor substrate; a gate dielectric formed on an upper surface ofsaid semiconductor substrate; and a conductive gate formed on said gatedielectric laterally disposed over a channel region of saidsemiconductor substrate, wherein said channel region extends laterallybetween a first source region of said semiconductor substrate and adetached drain region of said semiconductor substrate, wherein a channelboundary of said detached drain region is laterally displaced from afirst sidewall of said conductive gate by a drain displacement andfurther wherein a channel boundary of said first source region islaterally displaced from a second sidewall of said conductive gate by asource displacement; a first pair of spacer structures formed on saidfirst and said second sidewalls of said conductive gate, wherein alateral dimension of said first pair of spacer structures isapproximately equal to said source displacement; a first source impuritydistribution substantially contained within said first source region ofsaid semiconductor substrate; a second pair of spacer structures formedon exterior sidewalls of said first pair of spacer structures such thatexterior sidewalls of said second pair of spacer structures aredisplaced from respective sidewalls of said conductive gate byapproximately said drain displacement; and a drain impurity distributionsubstantially contained within said detached drain region of saidsemiconductor substrate.
 2. The transistor of claim 1 wherein saidsemiconductor substrate comprises silicon and wherein a resistivity ofsaid silicon is approximately 10 to 15 Ω-cm.
 3. The transistor of claim1 wherein said gate dielectric comprises oxide and wherein a thicknessof said oxide is approximately 20 to 200 angstroms.
 4. The transistor ofclaim 1 wherein said conductive gate comprises polysilicon and wherein asheet resistivity of said polysilicon is less than approximately 500Ω/square.
 5. The transistor of claim 1 wherein said conductive gatecomprises a metal selected from the group consisting of aluminum,copper, tungsten, and alloys thereof.
 6. The transistor of claim 1wherein a lateral dimension of said conductive gate is less thanapproximately 0.3 microns.
 7. The transistor of claim 6 wherein alateral dimension of said conductive gate is in the range ofapproximately 0.2 to 0.3 microns.
 8. The transistor of claim 6 wherein alateral dimension of said conductive gate is in the range ofapproximately 0.1 to 0.2 microns.
 9. The transistor of claim 6 wherein alateral dimension of said conductive gate is in the range ofapproximately 0.05 to 0.1 microns.
 10. The transistor of claim 1 whereina breakdown voltage of said transistor is greater than approximately 7V.
 11. The transistor of claim 1 wherein said source displacement isapproximately 50 to 400 angstroms.
 12. The transistor of claim 1 whereinsaid first pair of spacer structures comprises silicon nitride.
 13. Thetransistor of claim 1 wherein said first source impurity distributioncomprise ions selected from the group consisting of arsenic, boron, andphosphorous and further wherein a peak concentration of said firstsource impurity distribution is in the approximate range of 1×10 ¹⁷ to5×10²⁰ atoms/cm³.
 14. The transistor of claim 1 wherein said drainimpurity distribution comprises ions selected from the group consistingof arsenic, boron, and phosphorous, and further wherein a peakconcentration of said drain impurity distribution is greater thanapproximately 1×10¹⁹ atoms/cm³.
 15. The transistor of claim 1, furthercomprising a second source impurity distribution substantially containedwithin a second source region of said semiconductor substrate, whereinan interior boundary of said second source region is laterally displacedfrom said second sidewall of said conductive gate by approximately saiddrain displacement and wherein a peak concentration of said secondsource impurity distribution is approximately equal to a peakconcentration of said drain impurity distribution.
 16. The transistor ofclaim 1 wherein said second pair of spacer structures comprises oxide.17. The transistor of claim 1 wherein said drain displacement isapproximately 500 to 1500 angstroms.